1. Field of the Invention
The present invention relates to a process for the manufacture of SiC whisker-reinforced ceramic composite materials. More particularly, the present invention relates to a process for the manufacture of SiC whisker-reinforced ceramic composite materials having improved mechanical properties and process reproducibility by enhancing two-dimensional orientation of whiskers having geometrical anisotropy during hot pressing (sintering under uniaxial pressure).
2. Description of the Prior Art
SiC whisker-reinforced ceramics are composite materials which are useable as high-temperature structural materials as well as cutting tool materials.
Since, in hot pressing operation, pressure is applied in a uniaxial direction, the randomly-oriented whisker is subjected to rotational moment and tends to be arranged in a direction perpendicular to the applied pressure [G. C. Wei and P. F. Becher, "Development of SiC Whisker-Reinforced Ceramics," Am. Ceram. Soc. Bull., 64 [2] 298-304 (1985)]. However, the orientation of whiskers depend on packing density of a matrix material in a powder mixture, the amount of whisker added, aspect ratio and dispersibility of the whisker, etc.
Since the orientation of whiskers is likely to be restricted by direct contact with each other during hot pressing, it is very important in obtaining improved mechanical properties to add a maximum amount of whiskers having good dispersibility while minimizing the direct contact of whiskers before the hot pressing [J. V. Milevski, "Efficient Use of Whiskers in the Reinforcement of Ceramics," Adv. Ceram. Mat., 1[1] 36-41 (1986)].
Generally, it is known that the fracture of whisker-reinforced ceramic composite materials is caused by whisker clusters and large pores associated with the clusters, or whisker-deficient sites composed mainly of matrix material. That is, such destruction is mainly due to the aggregate of either whisker or matrix particles, which are produced owing to insufficient dispersion during the preparation of a mixed powder [P. D. Shalek et al., "Hot-Pressed SiC Whisker-Si.sub.3 N.sub.4 Matrix Composites," Am. Ceram. Soc. Bull., 65[2] 351-56 (1986)]. The mixed powder thus prepared is usually filled in granular form to a graphite die, and hot-pressed. The microstructure of the granules in the mixed powder will affect the process defect generation and the mechanical properties of the hot-pressed body as well as the behavior of the contact and rearrangement of whisker during the hot-pressing. Namely, when the whiskers in the granules have direct contact with each other, the network structure formed by the whiskers may retard the densification of composite materials by hot-pressing, resulting in the aforementioned whisker clusters; this results in the degradation of the physical properties of the composite materials.
We, the inventors, have made extensive studies to overcome the problems mentioned above and have found that SiC whisker-reinforced ceramic composite materials having improved mechanical properties and process reproducibility can be produced by enhancing the two dimensional orientation of whisker having geometrical anisotropy during the hot-pressing.